What Is a Polymer?
A polymer is a large molecule (macromolecule) built by joining many smaller repeating units called monomers. The word comes from Greek poly (many) + meros (part). Polymers can contain thousands to millions of monomer units, giving molecular weights of 10,000 to over 1,000,000 g/mol.
Polymers are everywhere in modern life — from the DNA in your cells to the plastic in your water bottle, from natural rubber to synthetic fibers. Understanding polymer chemistry helps explain why polyethylene is flexible while Kevlar is bulletproof.
Types of Polymerization
There are two fundamental ways to build a polymer chain: addition polymerization and condensation polymerization.
Addition (Chain-Growth) Polymerization
In addition polymerization, monomers with a double bond (usually C=C) link together without losing any atoms. The molecular formula of the polymer is simply n × the monomer formula.
Free Radical Addition Polymerization
The most common mechanism involves three stages:
Initiation: A free radical initiator (e.g., a peroxide, R–O–O–R) decomposes to form radicals. The radical adds to the double bond of the monomer, creating a new radical on the monomer.
Propagation: The monomer radical attacks another monomer's double bond, extending the chain. This step repeats thousands of times.
Termination: Two radicals combine (coupling) or one transfers a hydrogen to the other (disproportionation), ending the chain.
Important Addition Polymers
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Polyethylene (PE): Monomer = ethylene (CH₂=CH₂). The world's most produced plastic. High-density polyethylene (HDPE) has straight chains that pack tightly — used for milk jugs, detergent bottles, pipes. Low-density polyethylene (LDPE) has branching — softer, flexible, used for plastic bags and films.
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Polypropylene (PP): Monomer = propylene (CH₃CH=CH₂). Harder and stiffer than PE. Used in food containers, carpeting, automotive parts. Depending on the stereoregularity of the methyl groups, PP can be isotactic (ordered), syndiotactic, or atactic (random).
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Polyvinyl chloride (PVC): Monomer = vinyl chloride (CH₂=CHCl). Rigid PVC is used in pipes and window frames; plasticized PVC (with added plasticizers like phthalates) is flexible for tubing and flooring.
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Polystyrene (PS): Monomer = styrene (CH₂=CHC₆H₅). Transparent and rigid. Expanded polystyrene (Styrofoam) is 95% air, an excellent insulator.
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Polytetrafluoroethylene (PTFE/Teflon): Monomer = tetrafluoroethylene (CF₂=CF₂). Extremely chemical-resistant and low-friction surface. Used in non-stick cookware, plumber's tape, and laboratory equipment.
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Poly(methyl methacrylate) (PMMA/Perspex/Plexiglas): Transparent acrylic glass substitute.
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Poly(acrylonitrile-butadiene-styrene) (ABS): A terpolymer blend combining rigidity and impact resistance — used in Lego bricks and phone cases.
Cationic and Anionic Polymerization
Cationic polymerization uses Lewis acid initiators to create carbocation intermediates. Used for isobutylene → polyisobutylene (used in inner tubes).
Anionic polymerization uses organolithium or organosodium initiators. It produces very narrow molecular weight distributions (living polymerization) and is used for synthetic rubber (styrene-butadiene copolymers).
Condensation (Step-Growth) Polymerization
In condensation polymerization, monomers react through functional groups, and a small molecule — usually water or HCl — is released with each bond formed. Monomers must be bifunctional (have reactive groups at both ends).
Polyesters
Polyesters form when a diol (HO–R–OH) reacts with a dicarboxylic acid (HOOC–R′–COOH), releasing water:
HO–R–OH + HOOC–R′–COOH → …–O–R–OOC–R′–CO–… + H₂O
The most important polyester is poly(ethylene terephthalate) (PET): - Monomer: ethylene glycol + terephthalic acid - Uses: PET bottles (soft drinks, water), polyester clothing fiber (Dacron), food packaging
Polylactic acid (PLA) is a biodegradable polyester from lactic acid, derived from corn starch. Used in compostable packaging and medical sutures.
Polyamides (Nylons)
Nylon was the first fully synthetic fiber, developed by DuPont in the 1930s. Polyamides form from a diamine and a dicarboxylic acid (or from an amino acid):
Nylon-6,6: hexamethylenediamine + adipic acid → nylon + water. Used in stockings, toothbrush bristles, rope, and engineering plastics.
Nylon-6: caprolactam (a cyclic amide) undergoes ring-opening polymerization. Used in tire cords and textiles.
Kevlar: a rigid-rod aromatic polyamide (poly(p-phenylene terephthalamide)). The aromatic rings create a rigid chain, and extensive H-bonding between chains gives extraordinary tensile strength. Five times stronger than steel by weight — used in bulletproof vests, helmets, and aerospace composites.
Polyurethanes
Polyurethanes form from a diisocyanate and a diol: R–NCO + HO–R′–OH → urethane linkage (–NH–CO–O–)
Flexible polyurethane foams are used in mattresses and upholstered furniture. Rigid polyurethane foams provide insulation. Polyurethane coatings are used in floors, boats, and skis.
Polymer Properties and Structure
The properties of a polymer depend critically on its molecular architecture:
- Chain length: Longer chains generally increase strength and melting point. Molecular weight distribution affects processing.
- Branching: Linear chains pack tightly (higher density, higher melting point). Branched chains have lower density and more flexibility.
- Cross-linking: Covalent bonds between chains create a thermoset polymer — one that doesn't melt (e.g., vulcanized rubber, epoxy resin, Bakelite). Thermoplastics lack cross-links and can be melted and reshaped.
- Crystallinity: Semi-crystalline regions (where chains align) provide stiffness; amorphous regions provide flexibility.
- Glass transition temperature (Tg): Below Tg, the polymer is rigid and glassy; above Tg, it is rubbery. Processing temperatures depend on Tg and melting point (Tm).
Recycling and the Plastics Problem
Plastics are extraordinarily durable — a virtue in use, but a serious environmental problem at end-of-life. The global production of plastics exceeds 400 million tonnes per year, and much ends up in landfills or oceans.
Recycling codes (1–7) indicate the polymer type: - #1 PET: widely recycled - #2 HDPE: widely recycled - #6 PS: rarely recycled - #7 Other: almost never recycled
Chemical recycling (depolymerization back to monomers) is technically possible but economically challenging at scale. Biodegradable polymers (PLA, PHA) offer alternatives, but require industrial composting conditions to break down.
Green chemistry approaches focus on using renewable feedstocks (bio-based plastics), designing for recyclability, and developing catalytic processes that minimize waste.